This chapter is from the book

This chapter is from the book

MAC Addresses

A MAC address is a unique 6-byte address that is burned into each
network interface or more specifically, directly into the PROM chip on the NIC.
The number must be unique, as the MAC address is the basis by which almost all
network communication takes place. No matter which networking protocol is being
used, the MAC address is still the means by which the network interface is
identified on the network. Notice that I say network interface. That’s
very important, as a system that has more than one network card in it will have
more than one MAC address.

MAC addresses are expressed in six hexadecimal values. In some instances, the
six values are separated by colons (:); in others, hyphens (-) are used; and in
still others, a space is simply inserted between the values. In any case,
because the six values are hexadecimal, they can only be numbers 0–9 and
the letters A–F. So, a valid MAC address might be
00-D0-56-F2-B5-12 or 00-26-DD-14-C4-EE. There is a way of
finding out whether a MAC address exists through the IEEE, which is responsible
for managing MAC address assignment. The IEEE has a system in place that lets
you identify the manufacturer of the network interface by looking at the MAC
address.

For example, in the MAC address 00-80-C8-E3-4C-BD, the
00-80-C8 portion identifies the manufacturer and the E3-4C-BD
portion is assigned by the manufacturer to make the address unique. The IEEE is
the body that assigns manufacturers their IDs, called Organizationally Unique
Identifiers, and the manufacturer then assigns the second half, called the
Universal LAN MAC address. From the IEEE’s perspective, leaving the actual
assignment of addresses to the manufacturers significantly reduces the
administrative overhead for the IEEE.

CAUTION

As discussed, MAC addresses are expressed in hexadecimal format. For that
reason, they can only use the numbers 0–9 and the letters A–F. There
are only six bytes, so a MAC address should be six groups of two characters. Any
other number of characters or any answer that contains a letter other than those
described can be immediately discounted as an answer.

The method by which you can discover the MAC address of the network
interfaces in your equipment depends on which operating system is being used.
Table 3.5 shows you how to obtain the MAC address on some of the more common
platforms.

CAUTION

Be prepared to identify the commands used to view a MAC address as shown in
Table 3.5. You might be asked to identify these commands on the Network+
exam.

Table 3.5 Commands to Obtain MAC Addresses

Platform

Method

Windows 95/98/Me

Run the winipcfg utility.

Windows NT/2000

Run ipconfig /all from a command prompt.

Linux/Some UNIX

Run the ifconfig -a command.

Novell NetWare

Run the config command.

Cisco Router

Run the sh int <interface name> command.

As you work with network interfaces more, you might start to
become familiar with which ID is associated with which manufacturer. Although
this is a skill that might astound your friends and impress your colleagues, it
won’t help you with the Network+ exam. Just knowing what does, and
doesn’t, represent a valid MAC address will be sufficient on the exam.